Copper Zinc Tin Sulphide (Cu2ZnSnS4) Nanoparticle Ink Solar Cells

Abstract

Cu2ZnSnS4 (CZTS) quaternary semiconductor compound has potential properties for low cost thin film solar cells. It is composed of abundant elements and non-toxic material, with desirable properties for thin film photovoltaic (PV) applications such as high absorption coefficient close to 10-4 cm-1 and a band gap close to 1.5 eV. CZTS has been successfully fabricated by non-vacuum hot injection methods with a pure sulphur source. High concentrations of CZTS nanoparticle inks were deposited onto clean glass by spin coating techniques to study the CZTS nanoparticle ink quality.
This thesis reports the study of the influence of fabrication conditions for CZTS including temperature and time on structure and optical properties. This reveals that these temperature conditions 185, 205, 225, 245 and 265 oC with reaction times of 0.5, 1.0, 1.5 and 2.0 hours have strong effects on CZTS properties. The nanoparticles were deposited onto glass by the spin coating method. We report measurements made by UV-VIS spectrophotometer, Scanning electron microscopy, TEM, X-ray diffraction, energy dispersive X-ray spectroscopy and Raman spectroscopy. The results confirm energy band gaps decreases with increasing reaction temperatures. For increasing reaction times from 0.5 to 2.0 h the energy band gap increases with increasing reaction time. X-ray diffraction and Raman spectroscopy were used to study the structure of CZTS nanoparticles and show the crystal structure improves with increasing reaction temperature and times. It is also observed from SEM and TEM measurements that the size of particles increases with increasing reaction conditions. CZTS nanoparticles of differing composition (stoichiometric, Cu-poor/rich and Zn-poor/rich) are prepared to investigate the impact of composition on the properties of the thin films. Films are characterised using X-ray diffraction, electron microscopy, Raman spectroscopy and photoluminescence spectroscopy. Cu-rich sample has Cu2S secondary phases identified by a Raman peak at 475 cm-1. The result confirmed that the band gaps depend on the copper and zinc content as well as the particle size. The implications for PV devices are discussed. The results confirm that the optimum conditions for synthesising high quality nanoparticles are 225 oC for 1.0 h. this results in nanoparticles of dimension 35 nm with band gap of 1.5 eV, which are Cu-poor and Zn-rich.
The effect of thin film annealing parameters including temperature, time, ramping rate and atmosphere on the structure and optical properties are studied. XRD, Raman Spectroscopy, SEM and EDX are used to analyse films and demonstrate that the crystallinity of CZTS and homogeneity of elements improves with annealing conditions. This is an important factor for CZTS thin-film solar cells. The crystallinity, structure and chemical composition of CZTS thin film increased and improved under H2S+N2 atmosphere. It is concluded that annealing at 500 oC for 1 h with 10 oC/min under H2S (20%) +N2 (80%) atmosphere is a suitable condition for CZTS thin films used in solar cell devices.
CZTS solar cell devices which consist of substrate, back contact layer, followed by the main layer in this project; absorber layer, buffer layer, window layer, transparent oxide layer and grid layer were deposited as Mo foil/CZTS/ CdS/ i-ZnO/ ITO/ Al. Different thicknesses of CZTS layers were to study optical and electrical properties using Photoluminescence (PL) and I-V measurements. For PL measurements, the main peaks may correspond to CZTS in the range 1.35-1.55 eV. This study focused on the first four peaks, P1, P2, P3, and P4 at energies 1.38, 1.45, 1.55 and 1.63 eV for all samples. The most intense PL peak found at 1.55 eV is thought to be due to recombination of CZTS. The results confirm that all peaks have k-values less than 1. The values of k are in the range 0.75-0.92 ±0.02 which are related with radiative transition involving defect states. The PL peak energy has a blue shift to higher energy with increasing excitation power. However, a small peak shifts in the range between 1.0 and 4.0 meV/decade is seen in all samples. PL results confirm radiative transitions from defect states with electronic levels which are influenced by fluctuating potentials. This limits solar device performance. SEM images of devices show some unexpected features at the interface between CZTS and other layers: CdS and ZnO. This is shown to impact I-V characteristics. Damage in the device layers leads to loss of current collection and more investigations identified the source of the damage. Cross section images show uniform CZTS layers in the majority of devices. EBSD cross sections are used to identify the quality of crystal and phase orientation in the layer.